Theta Hya: Spectroscopic identification of a second B star + white dwarf binary

We report the identification, in an Extreme Ultraviolet Explorer (EUVE) spectrum, of a hot white dwarf companion to the 3rd magnitude late-B star Theta Hya (HR3665, HD79469). This is the second B star + white dwarf binary to be conclusively identified; Vennes, Berghofer and Christian (1997), and Burleigh and Barstow (1998) had previously reported the spectroscopic discovery of a hot white dwarf companion to the B5V star y Pup (HR2875). Since these two degenerate stars must have evolved from main sequence progenitors more massive than their B star companions, they can be used to place observational lower limits on the maximum mass for white dwarf progenitors, and to investigate the upper end of the initial-final mass relation. Assuming a pure hydrogen composition, we constrain the temperature of the white dwarf companion to Theta Hya to lie between 25,000K and 31,000K. We also predict that a third bright B star, 16 Dra (B9.5V), might also be hiding an unresolved hot white dwarf companion.Comment: 4 pages, 1 figure. Accepted for publication in Astronomy and Astrophysic

A Survey for Photometric Variability in Isolated Magnetic White Dwarfs—Measuring their Spin Periods

We present the initial findings of a photometric survey of isolated magnetic white dwarfs (MWDs) carried out with the 1.0m Jacobus Kapteyn Telescope. Of our sample of 30 MWDs, we have observed variability in 17 (57%) over our observed timescales (minutes to years), with a further 11 requiring more data, and two that are non-variable at the 1% level. In total we have discovered possible variability in 15 targets that has not been reported before in the literature, and we have measured the spin period of five objects in our sample to within a few percent. We find no correlation between spin period, mass or temperature, but there may be a weak negative correlation between period and field strength for the short-period targets. We have identified 14 MWDs with low field strengths and low temperatures, which are candidates for having star spots on their surfaces and should be followed up with polarimetry. We have also found that three low-field, high temperature MWDs are unexpectedly variable, with no obvious mechanism to cause this

A search for hidden white dwarfs in the ROSAT EUV survey II: Discovery of a distant DA+F6/7V binary system in a direction of low density neutral hydrogen

We report the results of our final search for hot white dwarfs in unresolved, Sirius-type, binary systems with IUE. One new system, RE J0500-364 (DA+F6/7V), has been identified. This star appears to lie at a distance of between 500-1000pc, making it one of the most distant white dwarfs, if not the most distant, to be detected in the EUV surveys. The very low line-of-sight neutral hydrogen volume density to this object could place a lower limit on the length of the Beta CMa interstellar tunnel of diffuse gas, which stretches away from the Local Bubble in a similar direction to RE J0500-364.Comment: 1 LaTex file plus 15 figures; accepted for publication in Monthly Notices of the Royal Astronomical Societ

Auroral Ion Upflows: Sources, High Altitude Dynamics, and Neutral Wind Effects

Large upwellings of thermal plasma are commonly observed in the high-latitude, topside ionosphere. These auroral ion upflows have a range of potential sources including frictional heating, electron precipitation, neutral winds, and higher-altitude density cavities. The unique signatures and detailed evolution of these upflows are examined through the use of Incoherent Scatter Radar data and a sophisticated ionospheric fluid model. A survey of solar cycle 23 shows that at Sondrestrom upflows occur most often in the cusp region and midnight auroral zone. Simplified force balance analysis and steady state velocity calculations are applied to a few select events to elucidate the role of the neutral wind in ion upflows. In some cases, the data suggests that neutral winds are necessary to balance the forces at lower altitudes. Detailed modeling shows that neutral winds will directly impact the efficiency of ion upflow mechanisms, and can create factors of ∼ 2-4 enhancements in upward ion fluxes in the topside ionosphere. Through detailed modeling, it has been shown that the commonly used steady state momentum equations are not consistently valid above ∼ 450 km. The significant transient effects, that exist at the high altitudes, imply that instantaneous input/output relationships for parameterizing ion outflow are likely inadequate. Steady state velocity calculations, in both radar data and simulations, tend to grossly over/underestimate speeds when the ions are accelerating/decelerating at high altitudes. A systematic simulation study of the efficiency and transient responses of the ionospheric upflow to various energy sources is also conducted. For this study, applied electric potentials were varied from 50 to 150 mV/m in 10 mV/m increments, electron precipitation effects peaking at a range from 2 to 20 mW/m 2 were varied in 2 mW/m2 increments, and density cavities were varied from 10% depletion up to 80% depletion in 10% increments. These results generally reveal that the propagation time delay between the F-region where the upflows are initiated and higher-altitudes is highly amplitude dependent. Electric fields exceeding 110 mV/m or particle fluxes exceeding 18 mW/m 2 create tremendous fluxes (10 13 m-3 s-1 ) of plasma that likely act as source populations for other energization processes above the ionosphere. Above 750 km, high altitude responses are not purely wave-like and include the dissipative effects of heat fluxes and heat exchange along with other complexities such as O + -H resonant charge exchange

Impacts of Anisotropy, Wave Heating, and Neutral Winds on High-Latitude Ionospheric Dynamics

Significant amounts of ionospheric plasma can be transported to high altitudes (ion upflow) in response to a variety of plasma heating and uplifting processes such as DC electric fields and precipitation. Once ions have been lifted to high altitudes, transverse ion acceleration by broadband ELF waves can give the upflowing ions sufficient energy for the mirror force to propel these ions to escape into the magnetosphere (ion outflow). In order to accurately examine the connection between upflow and outflow processes, a new two dimensional, anisotropic fluid model is developed. The new model, named GEMINI-TIA, is based on a Bi-Maxwellian distribution function and solves the time-dependent, nonlinear equations of conservation of mass, momentum, parallel energy, and perpendicular energy for six ion species important to the E-, F-, and topside ionospheric regions: O+, NO+, N+ 2 , O+ 2 , N+, and H+. Electrons have also been included using an isotropic description. The effects of photoionization, electron impact ionization, wave particle interactions and chemical and collisional interactions with the neutral atmosphere are included. In order to facilitate comparisons with data, the model accepts as inputs the main drivers of ion upflow and outflow: particle precipitation, electric fields, ELF wave power, and neutral winds and densities. GEMINI-TIA is used here in parametric and realistic case studies of ion upflow and outflow. In this research, GEMINI-TIA is first used in direct comparison with its parent isotropic model GEMINI to examine differences between isotropic and anisotropic descriptions of ionospheric upflow driven by DC electric fields. Further differences between isotropic and anisotropic descriptions of ionospheric upflow are examined through an additional comparison study that utilizes ionospheric drivers with realistic spatial and temporal variations. GEMINI-TIA, and its parent isotropic model GEMINI, are constrained by the MICA sounding rocket campaign data and respective outputs compared to analyze the impacts of anisotropy on low altitude ionospheric dynamics, specifically density cavity formation and related upflow. Next, GEMINI-TIA is used in a parametric study to examine ionospheric upflow driven by DC electric fields, possible effects of low-altitude wave heating, and impacts of neutral winds on ion upflow. Simulations show significant responses at low altitudes to wave heating for very large power spectral densities, but ion temperature anisotropies below the F region peak are dominated by frictional heating from DC electric fields. The time history of the neutral winds is also shown to affect the amount of ions transported to higher altitudes by DC electric fields and BBELF waves. Then, the role of neutral wind disturbances regulating ion outflow is further explored through model coupling between GEMINI-TIA and a neutral dynamics model guided by Sondrestrom ISR data. Specifically, a sequence of simulations with varying wave amplitude are conducted to determine responses to a range of transient forcing reminiscent of the ISR data. Thermospheric motions due to acoustic gravity waves (GWs) drive ion upflow in the F region, modulating the topside ionosphere in a way that can contribute to ion outflow. Lastly, GEMINI-TIA is used to study the spatiotemporal limitations of data driven modeling using the ISINGLASS sounding rocket campaign. Realistic variability of energy inputs into the ionosphere, from both the thermosphere and magnetosphere, are important when accurately determining the ion upflow/outflow response. Ground data driven simulations capture the shape and duration of ion upflows/downflows more accurately by containing both time and space variability but at the loss of the fine scale details that are present in in situ measurements. During the ISINGLASS campaign, the auroral arc had a pronounced southward drift, not captured in the rocket measurements, which slowly moves energization regions across the ionosphere generating a finite amount of heating in any given location. The overall ionospheric response, including the locations and strengths of upflows and downflow, is highly dependent on the time history of the ionosphere

Near-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG1234+482

We present a near-infrared spectrum of the hot ($T_{\rm eff}$ $\approx$ 55,000 K) DA white dwarf PG 1234+482. We confirm that a very low mass companion is responsible for the previously recognised infrared photometric excess. We compare spectra of M and L dwarfs, combined with an appropriate white dwarf model, to the data to constrain the spectral type of the secondary. We find that uncertainties in the 2MASS $HK$ photometry of the white dwarf prevent us from distinguishing whether the secondary is stellar or substellar, and assign a spectral type of L0$\pm$1 (M9-L1).Therefore, this is the hottest and youngest ($\approx 10^6$ yr) DA white dwarf with a possible brown dwarf companion.Comment: 5 pages, 2 figures, accepted by MNRA

Detection limits for close eclipsing and transiting sub-stellar and planetary companions to white dwarfs in the WASP survey

We used photometric data from the WASP (Wide-Angle Search for Planets) survey to explore the possibility of detecting eclipses and transit signals of brown dwarfs, gas giants and terrestrial companions in close orbit around white dwarfs. We performed extensive Monte Carlo simulations and we found that for Gaussian random noise WASP is sensitive to companions as small as the Moon orbiting a $V\sim$12 white dwarf. For fainter stars WASP is sensitive to increasingly larger bodies. Our sensitivity drops in the presence of co-variant noise structure in the data, nevertheless Earth-size bodies remain readily detectable in relatively low S/N data. We searched for eclipses and transit signals in a sample of 194 white dwarfs in the WASP archive however, no evidence for companions was found. We used our results to place tentative upper limits to the frequency of such systems. While we can only place weak limits on the likely frequency of Earth-sized or smaller companions; brown dwarfs and gas giants (radius$\simeq$ R$_{jup}$) with periods $\leq$0.2 days must certainly be rare ($<10\%$). More stringent constraints requires significantly larger white dwarf samples, higher observing cadence and continuous coverage. The short duration of eclipses and transits of white dwarfs compared to the cadence of WASP observations appears to be one of the main factors limiting the detection rate in a survey optimised for planetary transits of main sequence stars.Comment: 8 pages, 3 figure